Local Acceleration of Relativistic Electrons to Ultra-Relativistic Energy Due to Fast Magnetosonic Waves and Whistler-Mode Chorus Waves

The recent literature has related the acceleration of relativistic electron populations with moderate to intense geomagnetic storms. There is a consensus in the literature that the energization of 1–3 MeV electrons in the outer radiation belt results from adiabatic and non-adiabatic processes transporting low energy particles (tens to hundreds of keV) from the magnetotail through the inner magnetosphere in the radiation belt portion, and local acceleration due to whistler-mode waves, with a special contribution from chorus waves. However, the magnetospheric physical processes resulting in the ultra-relativistic (> 3 MeV) is still under debate. The acceleration of the later electron populations has been shown to be different from the former. Several works reported that a two-step process causes the enhancement of > 3 MeV electrons due to local acceleration related to whistler-mode chorus wave-particle interaction, followed by ULF wave inward radial diffusion. The studied events showed a good correlation between solar wind activity and the enhancement of the ultra-relativistic electron flux; however, the increase of ultra-relativistic electron flux is also observed during non-storm periods. In this work, we are interested in explaining these events' acceleration processes related to ultra-relativistic electrons. We report an unusual local fast acceleration due to whistler waves observed by Van Allen Probes. First, chorus waves enhance the 1–3 MeV outer radiation belt electron population, and then fast magnetosonic waves energize electrons to ultra-relativistic energies, such as higher than 5.0 MeV, under geomagnetic quiet conditions with a minor substorm. Electron phase space density analysis is compatible with local acceleration as the main mechanism participating in the ultra-relativistic electron appearance. Fast magnetosonic waves are concomitant with the> 5.0 MeV electrons enhancement, while chorus waves are concomitant with the increasing of the relativistic electrons. The wave normal angle, ellipticity, and planarity parameters measured by Van Allen Probes as a time function allow us to differentiate between the waves. Phase space density plots are applied to identify local acceleration signatures related to the wave's emissions. Finally, we calculate the resonance conditions for both whistler waves and obtain the corresponding resonant energy particle for each wave participating in this case study. Lastly, our analysis contributes to a better understanding of how fast magnetosonic wave acceleration can overcome whistler mode chorus waves to contribute to the ultra-relativistic electrons' appearance in the inner magnetosphere under geomagnetic quiet conditions.